1. Technical Field
The present invention relates in general to improved processing of air bearing surfaces and, in particular, to an improved system, method, and apparatus for multilevel UV molding lithography for air bearing surface patterning.
2. Description of the Related Art
Thin film inductive heads and magneto-resistive read heads require for their operation a very small constant effective magnetic spacing between their pole tips and the magnetic medium. This spacing has to be kept constant while the head travels with respect to the rotating disk at a relative speed of several meters per second. Air bearings have been found to be most efficient to guarantee this constant spacing even at variable operation modes caused by the head descending from the headrest or during large external acceleration. An air bearing is a carefully designed airfoil on the disk-facing side of the physical structure carrying both the read and write head. The disk-facing side of this structure (i.e., the slider) must be patterned with high accuracy so that the airfoil meets the required specifications.
The air bearing surface (ABS) includes rails and a cavity between the rails. The ABS is the surface of the slider nearest to the disk. Air is dragged between the rails and the disk surface causing an increase in pressure that tends to force the head away from the disk. Simultaneously, the air rushing past a depression in the ABS produces a lower than ambient pressure area at the depression. This vacuum effect counteracts the pressure produced at the rails. The opposing forces equilibrate so the slider flies over the surface of the disk at a particular fly height. The fly height is the thickness of the air lubrication film or the distance between the disk surface and the transducing head. This film minimizes the friction and resulting wear that would occur if the transducing head and the disk were in mechanical contact during disk rotation.
Typically, the air bearing is formed in the ceramic base material of the slider by two to three lithography steps, followed by either reactive ion etching (RIE) or ion milling (IM). In recent years, the process for industrial manufacturing of sliders has changed considerably and was driven by the shrinking dimensions of the read and write pole tips. To meet such tolerances, the pole tip surface (i.e., the slider lower surface) is lapped until the electrical and magnetic properties are within specification. Currently, lapping is either done on a solid “row” of about 40 to 60 sliders, a pre-parted “row” of about 40 to 60 sliders, or, most recently, on individual sliders. Each of these processes is followed by several photolithography steps to create the ABS on the lapped surface. The narrowing tolerance for the pole tips requires a more accurate lapping process despite the fact that rows of sliders are twisting and bending after being sawed off from the wafer. This problem is solved by partially separating individual sliders into short rows and by distorting the remaining row to allow optimal lapping of all sliders, or by fully separating the rows into individual sliders.
Since photolithography process cost is proportional to the resist coating, alignment, and exposure time, it is highly desirable to devise processes that can create the ABS surface of several sliders in a batch process. The current process, in which approximately 24 rows, each containing approximately 40 to 60 sliders, are mounted on a carrier and subsequently processed, is an air bearing patterning process called “Harmony.” The approach of mounting onto the carrier starts by placing rows or sliders with the pole tip facing downwards onto a sticky tape. The carrier is then covered with a UV-curable acrylate and put onto the slider assembly facing downwards. The sliders are fixed to the carrier by a thermal cure or a UV cure and the adhesive tape is peeled off. The lithography and etching process requires the embedding of the rows in a planarization layer, such as a polymer matrix material. For each photo-patterning step, a film resist is transferred to the slider array on the carrier and then exposed sub-row wise by projection lithography. The resist is then developed and the etching process is carried out on the entire slider array. After all processing is finished, the matrix material is removed and the sliders are released through a process called debonding.
In a process known as UV-imprint lithography or UV-molding, a patterned glass master or stamp is pressed into a liquid pre-polymer, which is then cured and solidified using UV light. After release, the pattern fixed in the polymer is an exact replica of the patterned glass surface except that it is difficult to fully displace a polymer on large areas to achieve a pattern with ideal contrast, which means that there is always a residual layer left. Use of an identically patterned elastomeric stamp instead of glass provided a similar replica but with two small differences. First, in the protruding areas of the stamp where the polymer was supposed to be completely displaced down to the supporting surface, thin droplets of polymer (also called “pancakes”) were trapped between the stamp and substrate. Second, variation was observed in the thickness of the features molded from the recessed zones of the stamp. Typically, the thicknesses of the features were smaller in the center of the features as compared to their periphery. The depths of these depressions were found to be proportional to the load applied to the stamp, a finding that allowed a mathematical model using equations of elastic theory to define and exactly describe this effect. Calculations for the displacement of the liquids were derived from lubrication theory.
Current hard disk drive technology is continually being pushed to higher levels of areal density and speed concomitant with downward pressures on cost. The ability to efficiently design, evaluate, improve, and subsequently fabricate air bearing sliders and suspensions is deemed critical for success in keeping up with these demands. Currently, patterning of the air bearing surfaces of read-write heads is a multi-step, photo-patterning process with limited yield and chronic reliability problems. Different approaches to the thin film resist technology exist either based on liquid resist, spray on resist, and resist film transfer that can do the required patterning. The difficulty arises from the challenging topography of arrays of slider bars or individual sliders, which require a planarization layer to complete the fabrication.